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In 2010, Michigan's Upper Peninsula recorded its first-ever earthquake when a large crack opened in the ground near the city of Menominee, according to a press release from Michigan Technological University. Now researchers finally know what caused the mysterious Menominee Crack. Sort of. Live Science reports the crack—360 feet long and 5 feet deep—showed up accompanied by a "deep boom" and the rattling of nearby homes. The earthquake that occurred along with it it measured less than a magnitude 1. For years, no one was sure what to make of the crack, which appeared atop a large limestone ridge. That area of Michigan is considered "aseismic," and there is no earthquake fault under the crack. Besides, any earthquake large enough to cause the crack would have done more than shake a few homes.

Researchers now believe the crack is a geological feature known as a "pop-up," according to a study published this week in Seismological Research Letters. Basically, the bedrock limestone "violently heaved upward," creating the bulge and a resulting crack as the surface stretched, Live Science explains. But that opens up an entirely new mystery. Pop-ups are caused when a large amount of downward pressure on the rock layer below the soil is suddenly lessened. So far, they've only been found at the bases of quarries and areas where glaciers have recently receded. But there are no quarries near the Menominee Crack, and Michigan hasn't seen a glacier in more than 11,000 years. "As far as we can tell, this is a one-of-a-kind event," research leader Wayne Pennington says in the press release. "The earth is still full of surprises," he tells Live Science. "It's just a little surprise, but it's still interesting." (A 30-year-old mystery was also recently solved.)